28 PROCEEDINGS OF MADISON MEETING. 



Localization of mineral Deposits by Faults. — In explaining the localization of deposits 

 by faults it is not necessary to suppose that the openings gave vent to mineral solu- 

 tions. A simpler explanation suffices, and the phenomena are better explained upon 

 the hypothesis that such linear breaks in the ocean-bed gave an outlet to fresh 

 water in large volumes from such a source as the Potsdam sandstone, or to carbu- 

 reted hydrogen gases or sulphureted hvdrogen, either of which would destroy 

 marine life and cause the accumulation of its remains in the sediments. Either 

 sulphureted gases or carbureted hydrogen would act directly upon sea water and 

 cause the precipitation of mineral sulphides, while the decomposing animal remains 

 would continue this reduction and cause the accumulation of pyritic ores in the 

 mass of the sediments in the vicinity of the faults. 



The hypothesis of the precipitation of metallic sulphides from sea- water by the 

 gases arising from decomposing animal and vegetable organisms was proposed by 

 Professor Whitney, but he did not entertain the idea of the source of such or other 

 precipitating gases in the faults, or of the possible destruction of organisms near 

 the faulting planes. The accumulation of such organisms near the faulting planes 

 would explain the localization of the mineral deposits if the precipitation of the 

 ores was dependent upon animal and vegetable decomposition. 



The phenomena of deposition of the sediments toward the close of the period of 

 the Trenton blue limestone are significant of a sudden incursion of muddy water 

 accompanied by petroleum. Although the upper layers of limestone are more 

 thinly bedded than the lower, so much so that they readily break up into tabular 

 masses and thin plates or flags, the rock retains its composition and dense homo- 

 geneous character. The remains of shells, however, become more numerous and 

 the surfaces of the flags are marked by very thin, shaly partings and many pro- 

 tuberances, consisting largely of the remains of brachiopods. 



Kelation op the Oil-rock to the Ore-deposits. 



In the so-called '' oil-rock " of the Shullsburg region we have ample direct evi- 

 dence of the sudden formation of hydrocarbons, and it does not appear that the 

 significance of this occurrence in connection with the mineral deposits has received 

 the amount of attention it deserves in any attempt to explain the phenomena of 

 the deposition of the lead and zinc ores. Professor Chamberlin has, however, 

 indicated the possible functions of the carbonaceous layers of the Trenton lime- 

 stone in causing the deposition of blende in the shale, and apparently after the 

 formation of the shale, the disseminated grains of blende, known to the miners as 

 " speckle jack," being found between the thin layers of the shale in such a way as 

 to indicate this.* 



This oil-rock is a true petroleum shale. A considerable quantity of thick petro- 

 leum can be distilled from it, even after it has been exposed to the weather for 

 years. It does not occur in quantity, but is generally in very thin layers not much 

 thicker than card-board or blotting-paper and film-like dividing laj^ers of compact 

 and highly fossiliferous limestone, but it sometimes attains a thickness of several 

 inches and is in several layers separated by limestone full of fossils. The color is 

 chocolate-brown, and is in strong contrast with the limestone, especially when 

 wet, by which the color is much darkened. It appears to have been suddenly 

 formed, the line between it and the limestone is sharp and distinct, and ' its posi- 



* Geology of Wineonsin, vol. iv, p. 546. There are numerous examples of this in the Shullsburg 

 region. Sometimes extensive workable beds are found, and the mass appears to have been formed 

 together. The blende is often rosettes or crystalline nodules—" strawberry " or " blackberry jack." 



